The measurement of electro-optic relaxation has often been used as a probe of the conformation and dynamics of DNA and other macromolecules. However, quantitative and full interpretations of such data for a flexible macromolecule like DNA have not been possible, primarily due to a lack of suitable (monodisperse) DNA preparations. The recent discovery of sequence specific DNA double strand cutting enzymes ("restriction" enzymes) enables us to now obtain monodisperse DNA samples which can be measured. Concomitantly the recent development in this laboratory of a new on-line electro-optic relaxation spectrometer has enabled us to begin a systematic study of the magnitude and effect of the binding of various agents to a) covalently closed, circular DNA and b) small monodisperse DNA fragments obtained by digestion of large molecules with site specific restriction enzymes. Our present experiments are on a mutagenic dye, ethidium bromide, which has already been extensively studied by other methods, and we are endeavoring to check out the intercalation/frame shift mutation model previously invoked to explain its biological activity. These results demonstrate the feasibility of investigating many other interesting macromolecular mechanism problems.